dominate xdm?

[richard <richm@isomedia.com>]

New install and just got X to work. xdm starts twm, which is lame, on
VC7 with a login. You can't get out of it on that console. Startx won't
work anywhere of cours.

I want to use e.g. icewm. How do I reconfigure xdm to default to a
different
window manager, and/or turn xdm off? I couldn't find the script to
change in /etc/X11.

I wrote a test .xsession file (did make executable) with just exec
xterm. Even though allow-user-xsession does in
/etc/X11/Xsession.options, that $HOME/.xsession doesn't prevail and
still stuck with twm.